There's an easier way. You can do this entirely in iWeb without having to change your code each time your publish your page. Thanks to an unnamed rep at PayPal who took the time to clear this up. I tried it tonight and it works fine.
To see it in action:
http://web.mac.com/jkevinwolfe/iWeb/PixTricks/Blog/Blog.html
The "Buy eBook" button is connected to PayPal.
Here's how you do it:
1. Go to Paypal's site and log in.
2. Click on the "Merchant Tools" tab.
3. Click on "Buy Now Buttons".
4. Enter the details of your sale.
5. Under "Choose a button to copy to your Web Site (optional)" leave the top "Buy Now" button checked.
6. For "Security Settings" Click "No" to encryption.
7. Click the "Create Button Now" button.
8. Copy all the text from the "Link for Emails" box.
9. Open iWeb.
10. Create a shape or text box with "Buy" (or whatever) in it.
11. Open the Inspector.
12. Click on the Links tab.
13. Check "Enable as a hyperlink".
14. Set "Link To" to "External Page"
15. In the URL box, paste the text you copied from PayPal.
When you publish your site the text field or button will now be follow normal paypal payment procedure. These instructions should work similarly for donation buttons.
Note: If you have Encrypted Website Payments turned on, you'll need to turn them off. Click My Account/Profile/Website Payment Preferences to find it. Safari seems to have a problem showing the Profile button. If it's invisible, drag your mouse over the top of the "B" in "Business Account Overview" and click.
Kevin